Pneumatic rubber tires, particularly high performance tires, are normally desired to have treads of a rubber composition which will provide good traction on the road.
An emphasis upon improving a traction characteristic of a tire tread rubber composition often compromises a tire tread's treadwear and/or rolling resistance characteristic as is well known to those having skill in such art.
For example, rubber compositions designed to specifically emphasize improved traction for a tire tread's running surface often present a relatively reduced abrasion resistance physical property and associated relatively greater treadwear property and typically exhibit a relatively high, single, glass transition temperature (Tg) of above −50° C. and usually within a range of about 0° C. to about −50° C.
Conversely, rubber compositions designed to specifically emphasize good resistance to abrasion for a tire tread's running surface often presents relatively reduced tire tread traction and also typically exhibit a single low Tg of below −50° C.
Therefore, a tread rubber composition exhibiting a desirable balance between traction and treadwear is difficult to achieve where the rubber composition exhibits only a single glass transition temperature (Tg).
In U.S. Pat. No. 5,723,530, it is mentioned that as tire is desired having a tread where good traction is desired yet still having an acceptable treadwear.
In U.S. Pat. No. 5,723,530, a tire is provided with a tread which is composed of four elastomers, of which two of the elastomers have clearly separated, therefore spatially defined, individual glass transition temperatures (Tg's). In particular the tread rubber composition is comprised of
(A) 25 to 60 phr of styrene/butadiene rubber with Tg in a range of −15° C. to −45° C.;
(B) 5 to 40 phr of medium vinyl polybutadiene rubber with vinyl content of 40 to 65 and a Tg in a range of −45° C. to −65° C.;
(C) 20 to 40 phr of cis 1,4-polybutadiene rubber with a Tg in a range of −95° C. to −105° C.; and
(D) 5 to 30 phr of cis 1,4-polyisoprene rubber having a Tg in a range of −65° C. to −70° C.
The Tg of the cis 1,4-polybutadiene rubber is required to be at least 50° C. lower than the Tg of the styrene/butadiene rubber. A carbon black with required Iodine and DBP (dibutylphthalate) values is also specified. It is readily seen in this patent disclosure that a tire tread rubber composition is provided which exhibits dual Tg's which contains less than 45 phr of cis 1,4-polybutadiene rubber. Thus, U.S. Pat. No. 5,723,530, while it is directed to a tread rubber composition which exhibits dual Tg's, it is not directed to a tire tread rubber composition which is comprised of a rubber composition which contains greater than 45 phr of cis 1,4-polybutadiene rubber.
U.S. Pat. Ser. No. 6,465,560 is directed to a cis 1,4-polybutadiene rich rubber composition for a tire tread rubber containing a minor portion of high styrene containing styrene/butadiene rubber with Tg's sufficiently spaced apart that the two elastomers are, basically, substantially incompatible with each other, as indicated by a plot of Tg versus temperature yields two distinct Tg maximums together with particulate reinforcement as an amorphous silica together with a high structure carbon reinforcement of specified carbon black Iodine value and DBP (dibutylphthalate) value characterization.
In the description of this invention, terms such as “compounded rubber”, “rubber compound” and “compound”, if used herein, refer to rubber compositions composed of one or more elastomers blended with various ingredients, including curatives such as sulfur and cure accelerators. The terms “elastomer” and “rubber” might be used herein interchangeably. It is believed that all of such terms are well known to those having skill in such art.
A reference to glass transition temperature, or Tg, of an elastomer or elastomer composition, where referred to herein, represents the glass transition temperature(s) of the respective elastomer or elastomer composition in its uncured state or possibly a cured state in a case of an elastomer composition. A Tg can be suitably determined by a differential scanning calorimeter (DSC) at a temperature rate of increase of 10° C. per minute (ASTM D3418-99), a method of determining a Tg of an elastomer which is well known to those having skill in such art.